In order to meet current emission regulations, active lean NOx catalyst systems with externally added reducing agents are used. In such systems, regulated emissions, such as certain nitrogen oxides, are reduced to nitrogen and water in the catalyst when a reducing agent containing hydrocarbons is added. In order to obtain maximum fuel economy while meeting emission regulations, it is necessary to inject just enough reductant to promote the chemical reactions without increasing hydrocarbon emissions by injecting too much reductant.
In certain circumstances, it may be desirable to have multiple lean NOx catalysts coupled in series to accommodate, for example, packaging and manufacturing constraints. In this case, dual reductant injection may be used for injecting reducing agent upstream of each catalyst.
One method for controlling reductant injection upstream of first and second lean NOx catalysts uses a NOx sensor located downstream of each catalyst. In this method, reductant is injected upstream of the first NOx catalyst in accordance with a control dependent on engine operating parameters. Similarly, reductant is injected upstream of the second NOx catalyst in accordance with a strategy identical to the first NOx catalyst. Both the reductant injection strategies rely on a downstream NOx sensor for the reductant control, and thereby the exhaust air/fuel ratio control. Such a system is described is U.S. Pat. No. 5,771,686.
The inventors herein have recognized a disadvantage with the above system. The above system does not indicate a method for determining the quantity of NOx entering the second catalyst. In addition, the above system does not exploit available benefits of having multiple catalyst with individual reductant injection control. In other words, the above method uses the same reductant control strategy for each catalyst, thereby requiring sensors for each catalyst. The inventors herein have recognized that improvements are possible by recognizing the physical couplings and eliminating unnecessary sensor duplications.